A clinch apex rubber is placed on a portion chafing against a rim of a pneumatic tire. The clinch apex rubber has functions for transmitting a driving force from the rim to the tire during driving and for holding the load of the tire. Therefore, the clinch apex rubber needs to have high hardness and superior heat aging resistance. Moreover, in order to reduce abrasion that occurs due to frictional contact with the rim caused by repetitive deformations of the during driving, the clinch apex rubber also needs to have a predetermined abrasion resistant property. Furthermore, since physical characteristics, such as rigidity, hardness and mechanical strength, of the clinch apex rubber give big influences to the steering stability performance during driving, these characteristics should be set within an appropriate range.
Along with the improvements of automobile performances in recent years, there also have been strong demands for high steering stability in tires. In order to obtain a tire having high steering stability, it is necessary to improve the rigidity of bead apex rubber. Conventionally, in order to improve the rigidity of the bead apex rubber, a large amount of carbon black is blended in a rubber composition for the bead apex.
To add a large amount of carbon black is effective to allow the bead apex to have high rigidity; however, this causes the problem that heat is easily generated in the tire during driving. When heat is generated, a fatigue resistant property of the tire is lowered to impair the durability, and a tire rolling resistance increases due to an increase in rubber loss tangent (tan δ) to cause an increase in fuel costs.
On the other hand, in recent years, environmental issues have been regarded as important, and regulations against CO2 emissions have been tightened. Moreover, petroleum resources are limited, and there is a possibility that a supply of materials derived from the petroleum resources, such as carbon black, might become difficult in the future, and at the same time, it is predicted that oil prices rises suddenly because of reducing the amount of the supply year by year. Therefore, there have been strong demands for replacing the materials derived from petroleum resources with materials derived from resources other than the petroleum resources.
At present, half or more of the entire weight of each of tires that are commercially available in general is composed of materials derived from petroleum resources. For example, since the general tire for a passenger car contains about 20% by mass of synthetic rubber, about 20% by mass of carbon black, a softening agent and synthetic fibers, about 50% by mass or more of the entire tire is composed of materials derived from petroleum resources. Therefore, there have been strong demands for the developments of a rubber for tires that is formed by using materials derived from natural resources, and satisfies required characteristics that are the same as, or superior to those of the rubber for tires made by using materials derived from petroleum resources.
Japanese Patent Laying-Open No. 2003-64222 (Patent Document 1) has proposed a rubber composition that contains 5 to 150 parts by weight of an inorganic filler, 0 to 30 parts by weight of a silane coupling agent and 5 to 100 parts by weight of vegetable fat and oil having an iodine value of 130 or less, relative to 100 parts by mass of diene-based rubber, in order to reduce the rolling resistance.
In order to provide a rubber composition for tread and a pneumatic tire that can greatly improve a wet-gripping performance without degradation in the tire abrasion resistance and rolling resistance characteristic, Japanese Patent Laying-Open No. 2002-105245 (Patent Document 2) has disclosed a rubber composition for tread which is composed of (A) 100 parts by weight of rubber components that include a diene-based rubber or a mixture of a natural rubber and a diene-based rubber, with at least 20% by weight of styrene-butadiene rubber being contained therein, (B) 5 to 50 parts by weight of clay, (C) 5 parts by weight or more of silica having a nitrogen adsorption specific surface area of 100 to 300 m2/g and (D) 1 part by weight or more of carbon black having a nitrogen adsorption specific surface area of 70 to 300 m2/g, with the total amount of (B) clay and (C) silica being set to 30 parts by weight or more and the total amount of (B) clay, (C) silica and (D) carbon black being set to 100 parts by weight or less.
In order to provide a rubber composition for base tread that can reduce fuel costs of a car during driving and a tire using such a rubber composition, Japanese Patent Laying-Open No. 2005-2287 (Patent Document 3) has disclosed a rubber composition for base tread which contains 1 to 20 parts by weight of a composite material composed of starch and a plasticizer, relative to 100 parts by weight of rubber components composed of a natural rubber and a butadiene rubber.
In order to provide a rubber composition for side wall that can reduce fuel costs of a car during driving and a tire using such a rubber composition, Japanese Patent Laying-Open No. 2005-53944 (Patent Document 4) has disclosed a rubber composition for side wall which contains 1 to 20 parts by weight of a composite material composed of starch and a plasticizer, relative to 100 parts by weight of rubber components composed of a natural rubber and/or an isoprene rubber as well as a butadiene rubber.
In order to provide a rubber composition for tires that can maintain performances required for tire members, such as air permeation resistance, flex cracking resistance and proper hardness, and also improves the processability, and a tire obtained by such a rubber composition, Japanese Patent Laying-Open No. 2006-89526 (Patent Document 5) has disclosed a rubber composition for tires which contains 30 parts by weight or more of silica, 5 to 15 parts by weight of calcium carbonate and 5 parts by weight or less of carbon black, relative to 100 parts by weight of rubber components composed of a natural rubber and/or its modified material.
In order to provide a rubber composition for breaker that is well balanced in performances, such as rigidity, heat resistance, adhesive property, adhesive property under wet and heat conditions and elongation performance, and a tire using such a rubber composition as a breaker layer or a belt layer, Japanese Patent Laying-Open No. 2002-338734 (Patent Document 6) has disclosed a rubber composition for breaker which contains 55 to 65 parts by weight of carbon black, 5 to 15 parts by weight of silica, 3.5 to 4.5 parts by weight of sulfur, 0.08 parts by weight or more of cobalt, a resorcine-based resin and methylene donator, relative to 100 parts by weight of rubber components mainly composed of a natural rubber and/or isoprene rubber.
However, the above-mentioned prior-art compositions have failed to provide a rubber composition for clinch apex that can reduce the amount of use of materials derived from petroleum resources, and is superior in crack resistant property and processability upon preparation, with a low heat-generating property, as well as a rubber composition for bead apex that has high rigidity and a low heat-generating property, and is superior in processability upon preparation, and a pneumatic tire using these.    Patent Document 1 Japanese Patent Laying-Open No. 2003-64222    Patent Document 2 Japanese Patent Laying-Open No. 2002-105245    Patent Document 3 Japanese Patent Laying-Open No. 2005-2287    Patent Document 4 Japanese Patent Laying-Open No. 2005-53944    Patent Document 5 Japanese Patent Laying-Open No. 2006-89526    Patent Document 6 Japanese Patent Laying-Open No. 2002-338734